Heat-resistant alloys



March 29, 1949. H. s. AVERY T RESISTANT ALLOYS HEA Filed Aug. 23, 1946 2Sheets-Sheet v1 BSN ATTORNEY INVENTOR. How/4R17 5. AVERY March 29, 1949.H. s. AVERY HEAT RESISTANT ALLOYS 2 Sheevs-Sheet 2 Filed Aug. 25, 1946FIG.2.

TEMPRTURE- .PFGS F//R IN VEN TOR. gow/4H@ SAl/EHY ATTORNEY Patented Mar.29, 1949 HEAT-RE SISTAN T ALLOYS Howard S. Avery, Mahwah, N. J.,assigner to American Brake Shoe Company, New York,

N. Y., a corporation of Delaware Application August 23, 1946, Serial No.692,589

This invention relates to high strength heat resisting alloys, and moreparticularly to iron base alloys including chromium, nickel, nitrogen,columbium and carbon.

An object of this invention is to provide a heat resisting alloycharacterized by high load `carrying ability a't elevated temperaturesparticularly inthe range from 1200 F. to 1800 F.

Another object of this invention is to provide l an alloy which isresistant to hot gas corrosion and which has .a low rate of deformationunder stress at high temperatures and a good life expectancy at highstresses.

Another object of this invention is to obtain alloys having highstrength and oxidation resistance at elevated temperatures with aminimum of such strategic elements as cobalt, tungsten, molybdenum,columbium, chromium and nickel. Of these, cobalt, tungsten and columbiumare quite expensive, and, if Irequired in large percentages, render theresulting alloy too costly for many industrial applications.

Iron base alloys containing substantial `amounts of chromium and nickelhave been generally found to be satisfactory for heat resisting service,the grade known as 26:12 (containing about 26% chromium Iand. 12%nickel, and known as Alloy Casting Institute Type HH) in particularAproviding perhaps the most economical corn-bination Iof hot gascorrosion resistance and elevated temperature strength. This grade, inthe form of castings, is usually lproduced within the range described byASTM Specica'tion B190-44T as follows:

claims. (o1. 'fs-12s) loys, by Howard S. Avery, Earnsh-aw Cook and J. A.Fellows, A. I. M. E. Technical Publication No. 1480, Metals Technology,August 1942, and Trans. A. I. M. E., vol. 150 (1942), pp. 373-400.)

For high service stresses, it is possible to provide greater `than usualstrength by manufacturing within the carbon range from 0.40% to 0.50%.However, the brittle nature of these alloys has led to their virtualabandonment in commercial practice. The brittleness of these alloys atroom temperature has been held responsible for service failures, but amore careful analy- V sis' and appropriate testing have indicated thattheir chief disadvantage is a short life expectancy at stresses imposed4to take advantage of their low deformation rates at high temperature.Stated differently, their creep strength is high,

but rupture strength and life expectancy .are relatively poor.

A survey of the 26% Cr: 12% Ni alloys described in the prior artindicates that they are limited to stresses below 4000 P. S. I. at 1800F. or fracture in less than 1000 hours may be expected, and similarlylimited to below 13,000 P. S. I. at 1400 F. If minimum deformation ratesof les-s than 1% per 10,000 hours are required the alloys are restricted:to below 3000 P. S. I. at 1800 F. and 10,000 P. S. I. at 1400 F.

In accordance with this invention, alloys are provided that have minimumcreep properties that equal or exceed the maximum creep properties ofthe prior art as ldescribed above. The alloys of this invention havecompositions including .4% to .8% carbon and preferably from Thecorresponding wrought alloys customarily contain lower chromium andcarbon contents. If the working stresses of such cast or wrought alloysare low, .they may be expected to give long satisfactory servi-ce. Ifservice stresses are high, however, they may soon become unservioeable'through excessive deformation (creep) or through fracture. over |a widerange [represented by limiting creep stress (L. C. S.) values from 300to 3,000 P. S. I. l at 1800 FJ by adjustment of the components of :thealloy. The chief factor for obtaining an L. C. S. of 4above 2200 P. S.I. at 1800 F. is carbon content. Below this level, the proper balance ofall alloying elements is helpful, the balance being judged by thefreedom from ferrite development in certain temperature ranges.(,Engineering Properties of Heat Resistant A1 Their strength is subjectto control .45% to .55% carbon, n-ot greater than 1.5% manganese andpreferably from .5% .to 1.5% manganese,not greater than 1.5% silicon'andpreferably .2% to 1.5% silicon, 13% to 22% nickel Y and preferably 13%t-o 16% nickel, 25% to 31% chromium and preferably 25% to 28%`chr-omium, .04% to .20% nitrogen and preferably .08% to .18% nitrogen,.85% .to 2.1% columbium and preferably 1.0% to 1.5% columbium and theremainder chiefly iron. It is essential that this alloy be austenitic;i. e., substantially free from ferrite. If this condition does notobtain, it is expected .to Ibe t-oo weak for the purpose intended. Toinsure production in the austenitic range, the alloys of this inventionshould have an alloy index not greater than 3.2 and preferably below 2.5when thel percentage of each of the constit- Alloy index: (.46 Cr-l-.TX

bon and nickel in the alloy in accordance with the following formula.:

The ,alloys .of this invention ,are characterized The ratio factoraccording to the authors of this by being substantially ausi-,animo andhave a permethod should be below 1.7, if a substantially maabjlity ofless than 1 05 They have an L C. S austenitic alloy is to be obtained.This formula (limiting .Creep Stress) ,at 1400 F. fat leastfails tolinclude the effect of other elements, par- 1o,co0 pounds per squareinch; an` L. C. s. at 10 ticularly, nitrogen.` and Consequently is validfor 1800" F. or at least 3,000 pounds per squareinch; only one; nitrogenlevel. It is also not correlated an L. R. s. (limiting rupture stress)for 1,000 with the.,y limiting creep strength over a range f hours at1400 F, 0f at 1eas t 13,000 pounds .pier the latter. It is useful,however, for determining square inch; and :an L. R.. S. for 1,000 hoursat if the limiting creepv stress at l800 F. is probably 1800 F. of atleast 4,000 pounds per square inch. 15 abQVe 0.1' belOW the 2000 poundsPer Square inch The alloys of this invention may be produced level.Moreover, the authors of this formula by conventional means in electricarc or inducpoint out that c arbon contents above .45% are tion furnacesif good steel making practice is fol- .llIlStSfdCtQry- While this ratiofaCtOr methOd lowed. The heats reported hereinafter were of, compositioncontrol may be used to produce made in basic lined furnaces, by charginglow-f alloys 0f abOut 2000 DOlmdS Per Square inch at carbon steel,ferrosilicon, nickel shot, low-carbon 18.00 F. or SQmeWhat abOVe, there.iS T10 evidence ferrochromium, nitrogen bearing ferrochroto indicatethat it makes consistently possible atmium, high-carbon ferrochromium,ferromangatdnmenlls 0f L- C- S- Values above 3000 pounds nese, andferrocolumbium in amounts calculated Der Square inch. WhChiS theapplOXmate maxitO yield the desired final analysis. The ferrothatldasbeenv attained in the prior art by manganese and ferrosilicon may bereserved unthe use of relatively high carbon contents. til after themelt down. Addition of the silicon as Itv 1135.106611, known QI Sometime that CarbOn 50%. ferrosilicon about two minutes before tap-`increases the strength of 26% Cr 12% Ni alloys. ping is generallysufcient for deoxidation. While This eiect is due partially toitsaustenite stabilfurnace practice is not critical, it is essential thatizlrly ilif,.lu.er10.e.4 but it also has an additional they skill of themelter be exercised to produce strengthening effectthat persists even inthesubclean, gas free metal. stantially. austeniticl compositions. Thus,in.-

, This material is intended primarily for cast-l creased carbon, mayconfer increased strength in ing. If hot working is desired, it must becaralloyshe permeability of whichis 1.00 and which ried out at a veryhigh temperature because of 0" contain no ferrite. Industrialexploitation has the greathot strength. ofthe` alloys. usuallyy beenconfined to the range from 0.25% to The data on elevated temperatureproperties 0.40%l carbon for general purposes, with occaof the specicalloys hereinafter described were sional use of the range from 0.4% to0.5% carbon obtained on as cast specimens, and the alloys for highstrength. The latter compositions are might be used in this conditoin.If undesirable appliedcautiously as their high resistance to de,-internal stresses areV` present after casting, they formationisassociated with relatively DOOr life may be partially relieved andreducedto tolerexpectancy, which prevents full exploitation of ablelevels by heating at 1800 F. for six hours, their strength. Alloyscontainingv above 0.50% followed by slow cooling. This treatment is red;carbon and the customary ranges of other alloy- COmmended if Criticalmachining is to bedone. ing elements` manifest poor elevated temperatureIf service temperatures below 1800? F. arev conductility,y short lifeexpectancy, and decreasing templated and dimensional stability. isimportant, creep strength. Consequently, suchl alloysv have an agingtreatment for twenty-four hours at the beenY considered undesirable,-and are seldom prointended service temperature.vv may be advisable.duced. commercially. Some early' attempts to This causes some carbideprecipitation andl con. utilize such materials led to disastrousfailures sequent contraction, but experimental evidence, because suchalloys did. notdeform in service but indicates that after twenty-fourhours dimensions failed by sudden brittle fracture. Current arerelatively constant. metallurgical literature and past experience with,

Several methods have heretofore been prothe addition of carbon beyondabout 0.40% have POSed fOr DIOdllCing a 26% Cr 12%. Ni alloy iI1-discouraged the formulationr of alloys containingv volving theutilization of a formula for the decarbon contents above this value. Theeffect of: termination of the alloy balance. One such carbon on 26% Cr12% Ni'alloys is indicated by method involves the amount of chromium,carexperimental'evidence in Table 1.

TABLE 1` Eects of carbon on 26.3% Cr 11.3% Nri heat resistant allloysRoom Temperature Proi L C S porties After Aguing for Telllggg 18000.F 24hrs.- at 1400 F. Dummy o 00017, Furnace. Cooling o F. C. M11., Si. Ni,or, N, H 1y at 1400v Alloy percent percent percent percent percentpercent Der; -r 201000 p- .s' l' (1% per Elon amm Elonganon 110,000Hats.) Tensile in egrcent in percent;`

p, s, 1., Strength, forllzmch for 2 inch p. s. gauge 'length gaugelength 0.19 41 43v 11. 4 26. 5v 08@ 600. e 000 30, 0 26.5 0. 31 42 4211. 4 26. 5 07 1150 86,V 000v 25.` 0 20:0 0.42 .45. 45 11:3 26. 1: 06;2200' 89,00() 12.15' 9. 5 0. 52 53 47 11. 4: 26. 4 07 2650, 9,5l 750 4.02.,5 0.61 51 48 11. 4 26. 3' l .07 2500' 93; 900 2. 8 1. 5

Elongation per cent in 2 These recom- Greep Rate per cent Hr.

04.56 40213 7w257839um0913741ww32211 5.3.7.LQm5.0.0.0.Hm2.L0.0.0.0.0.0.0.0.000

Hours 1800 F.-Stress-Rupture8000 p. s. i.

' Time,

776012686424 .L .1 36 22955 1022531492 41UM22n22233 Mul The authors ofthis proposal, however,

Alloy Index H.=24 FractureA 3683059137309853977051522.omL5.L2.2.2.2.LA3.2.L2.2.L0.L2.LL20

This invention'differs from the prior art in retween 0.85% and 2.1%.

Typical examples of alloys of this invention,

Cb, per

cent

mendations are coupled with the restriction of carbon plus half of thenitrogen to the preferred range from .26% to .38% and a maximum of0.45%.

note significantly that this use has little or no influence on thestrength properties of the base alloy.

quiring the simultaneous combination of three factors in the HH type ofalloys:

1. An alloy having a carbon content from 0.4%

2. A balanced substantially austenitic alloy, and

so 3.. An alloy having a columbium content of be- Any two of thesefactors` are not suilicient to produce consistently the high strengthand life expectancy that represent the characteristics deincluding theirchemical composition and mechanical properties, are given in Tables 2and 3`,

which also indicate compara-ble properties of al- TABLE 2 N,per centCreep characteristics at 1800" F.

AHOY v Ceelir The sistant alloys cont . Ni,per Cr,per

cent

Chemical Analysis Si, per cent i Mn, per cent .LCL

While nitrogen, like carbon, strengthens the 26% 15 0.08% to 0.16% lowercarbon. Cr: 12% Ni alloys, large amounts of it tend to It has beenproposed that columbium and silicon be. added in combination. to promoteformation of a sigma phase which may produce precipitation hardeningafter suitable heat treat- Columbium has been proposed as an additionStress-rupture life and secondary creep ratle comparison of 'various 26%Cr 12 Nz' type heat re-- agent to neutralize or minimize the tendency ofS11-edm the auoys of this invern-,ion carbon to lower the ductility ofthe alloy as cast and after exposure to elevated temperatures,

about 0.5% to 1% columbium being employed to produce room temperatureductility characteristics similar to those of an alloy With perhaps 40loys outside the scope of this invention.

promote formation of a lamellar constituent which may be associated withhigh short-time strength but relatively low creep strength.

ments. This condition is usually associated with brittleness atatmospheric temperatures, and, because the sigma phase tends totransform to ferrite at high temperatures, this mechanism is notconsidered valuable for producing alloys having high strength atelevated temperatures. HHB'IFS alloy listed in Table 2, infra, is acomposition of this nature and demonstrates the rel atively low strengthof this combination.

Alloy Index :I: itat... uwwmww .mmmmmm HHHHH 1 Permeability determinedafterwater quenching from 24* hours at 2000"4 F;

Tanz.: 3

Limiting creep stress and limiting rupture stress comparisons at 1400 F.and 1800" F. for various 26% Cr 12% Ni type alloys chemins AnalysisLimgssqeep y Lmntpm Alloy C, per Mn, per Si, per Ni, per Cr, per N, perCb, per s o o o cent cent cent cent cent cent cent 1400 F 1800 F' 1400F' 1'800 F 32 48 46 11. 5 25. 9 27' 51 1. 18 12. 0 25.0 2, 250 45 54 6012.8 26. 9 2, 750 .31 .53 .66 10.7 27.1 19 41 43 11.4 26. 5 l, 500 .31.42 .42 11.4 26.5 2, 200 42 45 .45 1l. 3 26.1 3, 800 52 53 47 11. 4 26.4 3, 000 6l 51 48 11. 4 26.3 2, 800 41 .54 56 14. 2 26.7 4, 900 48 50 5814. 1 26. 6 4, 800 .63 .48 .49 14.0 26.6 4,500 65 56 47 14. 0 26. 5 4,700 77 46 48 13. 9 26. 4 5, 100

l For a minimum or secondary stage creep rate below 0.0001 per cent perhour; Stress in p. s. i. 2 For a rupture time or life expectancy of1,000 hours; stress in p. s. i.

The last ten alloys listed in Table 2 and the last ve alloys listed inTable 3 are typical examples of the alloys of this invention, and areproduced by the method heretofore described.

In Table 2, the alloy index is calculated in accordance with the formulanoted above and is given for each alloy. The permeability of each alloyis expressed by the Greek letter mu (il). The stress-strain-rupturedeterminations of these alloys are expressed in terms of fracture timeor hours under load before rupture. More particularly, these =valuesindicate the time, in hours, required to fracture the alloy at 1800 F.under a stress of 8000 pounds per square inch. The column captionedCreep rate in per cent per hour is the minimum or secondary stage rateof deformation that is characteristic of creep tests. The columncaptioned Elongation is the percentage increase in length over atwo-inch gauge length during the period under stress before fractureoccurred. It is a measure of the elevated temperature ductility of thematerial. The columns captioned Limiting creep stress, in Table 3,represent the stress required to produce a minimum or secondary stagecreep rate not greater than 0.00 l% per hour elongation at 1400 F. or1800 F. respectively. These values are derived from the interpolation orextrapolation of several creep and stress-strain-rupture tests plottedlogarithmically as shown in Fig. l of the drawing. They correspond to acurrent engineering definition of limiting creep stress for anelongation rate of 1% in 10,000 hours; The columns captioned limitingrupture stress indicate the stress in pounds per square inch that isexpected to produce fracture in 1000 hours and is obtained byinterpolation or extrapolation of the fracture times from two or morestress-strain-rupture tests plotted logarithmically as illustrated inFig. 1. The extension of the fracture time plots permits the estimationof maximum life expectancy at any given stress by reading the stresscoordinate that intersects this fracture time line. An example of thecharacteristics at elevated temperature of a typical alloy of thisinvention is shown in Fig. 1, representing alloy I-IH41Cb in Table 2.This alloy is near the low side of the range contemplated by thisinvention.

Figure 1 is a convenient summary of elevated temperature properties asderived from stressstrain-rupture and creep tests. These two areessentially the same except that creep tests continue for comparativelylong times under relatively low loads and usually do not producefracture of the specimen. The stress-strain-rupture tests are scheduledat higher loads and are intended to produce fracture within reasonabletime intervals (e. g. up to six weeks). The creep rates of both of thesetests are comparable in signicance though different in magnitude. 30They apparently are related in such fashion that the data from severalplot very close to a straight line on logarithmic paper. The lines inFigure l that slope downward to the left connect the creep rate pointsobtained by testing at the loads of 30,000 pounds per square inch,20,000 pounds per square inch and 10,000 pounds per square inch at 1400F., and 8000 pounds per square inch, 6000 pounds per square inch and5000 pounds per square inch at 1800" F. Plotting in this fashion permitsthe estimation of creep rate at any stress not toc far removed fromthose of the actual test. The intersection of these lines with thatdesignated 0.0001% per hour creep rate determines the limiting creepstress as previously defined.

The fracture times derived from the same tests described above aresimilarly plotted to produce the lines sloping downward to the right.They permit estimation of the life before fracture at any stress withinthe range of those employed and also permit an approximate prediction ofbehavior under stress for time intervals too long to be practical foractual testing. It should be recognized that there is an element ofuncertainty in the extrapolation of the fracture time lines to intervalsbeyond about 2000 hours. Unfortunately, no better method is availablefor predicting engineering performance without actually conducting thevery long-term tests. It obviously is not practical to extend the timeof a creep test under the stress of 7800 pounds per square inch at 1400F. to determine if the specimen actually lbreaks at the end of 100,000hours, as this test would require twelve years for completion. Thus,these extrapolated values are not to be considered as completely certainpredictions of performance, but they do serve the very useful purpose ofpermitting the comparison of various alloys on the same basis and underconditions that most nearly approximate the requirements of service.

It may `be noted that the values for life and creep rate as recorded foralloy HH41Cb in Table 3 appear on the logarithmic plot as shown inFigure 1. The limiting creep stress and limiting rupture stress valuesin Table 3 are designated by arrows on Figure 1. The other values forlimiting creep stress and limiting rupture stress that appear in bothTables 2 and 3 were derived from plots similar to these.

The engineering properties of the alloys of this invention having markedstrength at elevated temperatures are shown graphically in Figure 2,which is a convenient summary of elevated temperature properties asderived from data exemplied in Figure 1. It permits the selection ofrecommended design stress, limiting creep stress, or limiting rupturestress values at any temperature between the limits of 1400 F. and 1800F. On the same charts are plotted the values for yield strength andultimate tensile strength as obtained from conventional tests atelevated temperature. These values are less useful in design, but aresometimes requested by engineers. If parts of these alloys are subjectedto hindered contraction, large thermal stresses may develop. 'I'hesewill rapidly decrease because of creep to values that are characteristicof the temperature involved and which may be derived from the linelabeled Hindered contraction stress on the diagram. This same line alsoindicates the approximate residual stress level that will result fromstress relief annealing for a short period at any temperature within therange indicated. A further description of the significance of thehindered contraction stress is included in A. I. M. E. technicalpublication No. 1480, entitled Engineering Properties of Heat ResistantAlloys, by H. S. Avery, E. Cook and J. A. Fellows. It will be noted thatthis diagram is drawn on semilogarithmic coordinates, as this permitsshowing the temperature-stress relationships by means of straight linesover the range from 1400 F. to 1800 F.

The lines on the diagram of Fig. 2, with the exception of those labeledYield strength and Ultimate tensile strength, represent approximateminimum values of the alloys of this invention, and may be employed fordesign purposes with reasonable assurance of their validity over a widecomposition range, provided the restrictions of this invention areadhered to. The lines for yield strength and tensile strength representaverage values and are bordered by a stippled zone to indicate theapproximate range in these properties that was encountered duringtesting.

The room temperature mechanical properties and thermal expansioncoefficients of the alloys4 of this invention are detailed in Tables 4and 5.

TABLE 4 Room temperature mechanical properties balance within thespecied limits.

The usual -commercial 26% Cr 12% Ni alloys have good oxidationresistance up to 2000 or 2100 F. The columbium-bearing alloys of thisinvention have less resistance to hot gas attack, and a maximum servicetemperature of 1800 F. is suggested. If it is proposed to exploit itshigh strength above 1800 F., it is desirable to increase the chromiumcontent in proportion and use the upper part of the range provided.Oxidation resistance can be increased by additional silicon also. Theuse of silicon above 1.5% may be required, however, which tends toimpair hot strength.

Unless superior oxidation resistance is especially required, it isrecommended that production be conned to the range from 25% to 28%chromium. Chromium below 25% may be satisfactory for strength, butresults in decreased protection against hot gas attack.

For alloys of this invention, caution is required in the repair orfabrication of parts by fusion welding. The high hot strength minimizesthe plastic deformation that operates to relieve the high thermalstresses developed by the sharp temperature gradients of welding. Weakermaterials thus have less tendency to crack. Minor welding repairs may bemade with a weaker, more plastic alloy, such as American Iron and SteelInstitute type alloy 309Cb, containing about 0.80% columbium and 10Wcarbon, but hot strength will be sacrificed for freedom from cracking.If the weld metal is to be critically stressed and must have thestrength of the casting, it should be made to the same specification.The avoidance of cracking will then depend on the reduction of thermalgradients by preheating, slow welding and other control factors known toWelders.

The alloy index formula in accordance with this invention may lead tothe inference that the various elements may be substituted for eachother within its quantitative limitations. This is true only forcontrolling the ferrite-austenite A low carbon alloy, as HH31Cb, inTable 2, may be entirely austenitic but nevertheless exhibit hotstrength below the minimum required. Thus increased nickel is not asatisfactory substitute for lowered carbon.

Nitrogen is more nearly equivalent to carbon in its strengtheningfunction. Comparison of alloys I-IH38A with HH42 and HH45A in Table 2illustrates this in the absence of columbium. With about 1% columbium,I-IH38Cb versus HH41Cb and HH48Cb permits a similar comparison and alsodemonstrates the marked strengthening associated with the columbiumaddition.

In the production control of this high strength alloy, it is intendedthat the alloy index formula be employed prior to melting and for moltenbath adjustments after preliminary chemical analysis. After productionchemical analysis may be used to estimate hot gas corrosion resistance,the 25% Cr minimum being important, to detect borderline heats that maybe partially ferritic, and to establish that the desired carbon minimumis attained. For a more precise check of the austenite balance arepresentative sample is held at 1850" F. for twenty-four hours, waterquenched, and a 0.250 inch diameter x 1.00 inch long specimen machinedfrom it, being iinished preferably by grinding. This is checked in apermeameter as described in Precision in Creep Testing, A. I. M. E.Technical Publication No. 1MB-C (1942), No. 303, by J. A. Fellows, E.Cook and H. S. Avery,

1l with a magnetizing force of about 24 oersteds; if the permeability isbelow 1.05, the alloy balance is considered satisfactory.

Precise control of chemical composition, especially near the extremelimits of the constituents of the alloys of this invention, is a vitalfactor in producing these alloys satisfactorily. As the introduction ofcolumbium into the chromium-nickel-lron alloys may contribute somedifculties in chemical analysis, the following references are includedto permit duplication of the results of this invention.

Metals Handbook, 1939, published by the A. S. M.

Analytical methods for:

Carbon, pages 684-685 Manganese, pages 686-687 Silicon, pages 692-693Chromium, pages 698-699 Nickel, pages '700-'701 Columbium, pages'708-709 Scotts Standard Methods of Chemical Analysis, fifth edition,vols. I and II. Published by D. Van Nostrand Company.

Carbon, page 1430 Silicon, page 1446 Nickel, pages 1452-1453 Chromium,pages 1445-1446 Chemical Analysis of Iron and Steel, Lundell, Hoffman &Bright, 1931, published by John Wiley & Sons.

Carbon, page 171 Manganese, pages 193-194 Silicon, page 261 Nickel,pages 280-285 Chromium, pages 291-292 Nitrogen, page 426 A'. S. T. M.Methods, 1936.

Carbon, pages 9-10 Manganese, pages 14-15 Silicon, page 27 Nickel, pages31-36 Chromium, pages 36-38 Determination of Soluble and InsolubleNitrogen in Ferrochromium Corrosion and Heat Resistant Alloys, by C. M.Johnson, in Iron Age, July, 1935. Reproduced by Precision Scientic Co.,Chicago, Illinois, in their Bulletin 151A.

It has been found during years of research on these heat resistingalloys that confusing and misleading results are obtained if thechemical analyses reported are not characterized by accuracy and highprecision. With very careful attention to details of precedure andtechnique it is probable that the essential elements in these alloys canbe determined within the following limits: Carbon 10.01; manganese10.03; silicon 10.04; nickel 10.15; chromium 10.20; nitrogen 10.005;columbium 10.03. To obtain this precision requires the averaging ofduplicate determinations. The eifect on the alloy index number issufficient, if all deviations should be in direction of maximum effect,to produce a range of 10.36. This range, while not desirable, is not tooserious as it is considered to be of approximately the same magnitude asthe uncertainties in the alloy index formula. However, if analyses arecarelessly run or if -data are contributed from a number of sources, theerror may be much more serious. As an example: A group of analysesreported from various sources on a number of samples, there beinggenerally four different reports on each sample, covered a range indeviation from the average of 0.01% to 0.20% carbon; 0.01% to 0.61%manganese; 0.01% to 0.65% silicon; 0.10% to 5.5% nickel; 0.20% to 4.2%chromium; and 1.02% nitrogen. If these ranges had been employed in alloyindex calculations, the variation would have been plus or minus 1.19 inthe A. I. number. Some of the reports were obviously out of line withthe others; rejection of these, which was made possible by theavailability of several analyses, reduced the alloy index spread to plusor minus 0.54.

A recognition of this problem in precision of analysis was included in apast specification for heat resistant alloys, which permitted adeviation on recheck analysis of 1.02% carbon; 10.1% manganese; 10.1%silicon; 10.4% nickel; 10.6% chromium; and 10.02% nitrogen. Thislatitude would permit variations of plus or minus 1.02 in alloy indexnumber. The above data have been presented to emphasize the necessityfor knowing the real chemical composition of the alloy if proper controlis to be exercised in production to obtain the strength values of whichthe material is capable.

The terms and expressions which I have employed are used as terms ofdescription vand not of limitation, and I have no intention, in the useof such terms and expressions, of excluding any equivalents of thefeatures shown and described or portions thereof, but recognize thatvarious modications are possible within the scope of the inventionclaimed.

What is claimed is:

1. A heat resistant alloy having the composition comprising .4% to .8%carbon, not greater than 1.5% manganese, not greater than 1.5% silicon,13% to 22% nickel, 25% to 31% chromium, .04% to .20% nitrogen and .85%to 2.1% columbium and the remainder essentially iron, said alloy beingcharacterised in having an alloy index as calculated in accordance withthe following formula, of not greater than 3.1:

Alloy index= (.46X Cr .7X Sl+ .2 Mn+1.3 Cb)-(.56 Ni+ 6.2 C+10.0 N) inwhich formula each constituent in the alloy is indicated by itspercentage of the total alloy.

2. A heat resistant alloy having the composition comprising .45% to .55%carbon, .5% to 1.5% manganese, .2% to 1.5% silicon, 13% to 16% nickel,25% to 28% chromium, .08% to .18% nitrogen, and 1.0% to 1.5% columbiumand the remainder essentially iron, said alloy being characterized inhaving an alloy index, as calculated in accordance with the followingformula, of not greater than 3.1:

.2 Mn+1.3 Cb)(.5e Ni+ 6.2 C+10.0 N) in which formula each constituent inthe alloy is indicated by its percentage of the total alloy.

3. A heat resistant alloy having the composition of about .48% carbon,about 26.6% chromium, about 14.1% nickel, about .12% nitrogen, about .6%silicon, and about 1.00% columbium and the remainder essentially iron.

4. A heat resistant alloy having the composition .4% to .84% carbon,about 26.3% chromium, about 14% nickel, about .5% silicon, about .11%

nitrogen, and about 1% columbium and the remainder essentially iron.REFERENCES CITED 5. An iron base heat resisting alloy of the 26% Thefollowing references are of record in the Cr 12% Ni type including 0.40%to 0.80% carle 0f this patenti bon, 0.04% to 0.20% nitrogen and .85% to2.1% 5 columbium, said alloy exhibiting after a heat UNITED STATESPATENTS treatment of twenty-four hours at 1850 F. fol- Number Name DateloWed by water quenching a magnetic permeabil- 2,174,025 WiSe et al Sep.26, 1939 ity not greater than 1.05%. 2,223,659 Harder et al Dec. 3, 194010 2,256,614 Franks Sept. 23, 1941 HOWARD S. AVERY.

